In display devices such as LCD that utilize backlighting without emitting their own light, the backlighting usually consumes most of the electrical power. In such cases, lowering the power consumed by backlighting is the key to lowering the total power consumption in the display device.
Attempts were therefore made to lower power consumption in the display device by lowering the light intensity of the backlight in dark image scenes. Simply lowering the light intensity of the backlight to 1/N also lowers the screen brightness to 1/N. However, if the transmittance of each liquid crystal pixel could be increased N times by correcting each pixel value, while also lowering the light intensity of the backlight to 1/N, then a final screen brightness can be maintained.
The transmittance of each liquid crystal pixel cannot however be a larger value than the maximum possible transmittance of the liquid crystal element. The N value therefore has an upper limit. Setting N to a maximum within a range that will not deteriorate the image quality requires adjusting the N value so that the liquid crystal pixel transmittance of the brightest pixel in the display image is the maximum pixel transmittance. This method for collectively controlling the backlight luminance value on the entire screen is called global dimming.
However if there is a luminescent spot on even just one point on the screen during global dimming, then the entire backlight luminance rises because the N value cannot be increased by this luminescent spot. This rise in backlight luminance sometimes suppresses the power saving effect due to the particular image content.
Due to this problem, in recent years much attention is focusing on methods called area control or local dimming that control the backlit luminance in each area by dividing the screen up into small areas and utilizing a light source matching each separate area to autonomously control the light emission intensity in each light source. In this method, the light emission intensity of the corresponding light source in each area is set based on the pixel values in that area using the same method as in global dimming. Applying this method to all areas within the screen sets the light emission intensity all the light sources. Along with using these values to control all light sources, each pixel value for the input image can be corrected the same as for global dimming to allow lowering the electrical power consumption with almost no loss in image quality.
The display luminance can in this way be maintained by correcting each pixel value along with reducing the backlighting by area control to boost the transmittance of the liquid crystal element. The relation between liquid crystal transmittance and each pixel is generally a power characteristic called the gamma characteristic dependent on the liquid crystal panel. The area control in other words, corrects the liquid crystal element transmittance according to the backlight fading rate and sets the final pixel value from this transmittance and the liquid crystal panel gamma characteristic. So the panel gamma characteristic is preferably unchangeable during area control.
Fluctuations in the gamma characteristic however are unavoidable according to the viewing direction relative to the actual liquid crystal panel. In this case, correcting the image based on the gamma characteristics when looking at the screen from the front may cause a strange impression if the screen is viewed from the side.
A method to alleviate this problem by limiting the amount of spatial change in backlight luminance was proposed in Japanese Patent No. 4235532. However, this method reduces the backlight fading rate which suppresses the effect that lowers power consumption.
In many cases this method provides images with almost no strange impression when viewed from the side after applying area control. Even in these images however, the processing reduces the backlight fading rate, which in turn suppresses the effect that cuts power consumption.